A variable speed water-to-water heat pump model used for ground-source applications

ABSTRACT: The main objective of this study is to model variable speed water-to-water heat pumps (VSHP) and to examine the impact of the operation of such devices on ground heat exchanger sizing and energy consumption when they are used in ground-source applications. In the first part of the paper, a complete physics-based steady-state model of a variable-speed water-to-water heat pump is briefly presented. A performance map approach is also used by modifying an existing TRNSYS variable speed heat pump model to provide a minimum speed of operation and a better representation at part load. Simulation results over a heating season on a residential ground source VSHP indicate that the energy coverage (i.e. percentage of annual heat supplied by the heat pump) increases at a faster rate than the effect coverage (i.e. percentage of peak building heat supplied by the heat pump) for a small value of the effect coverage. For example, for an effect coverage of 60%, the energy coverage is ∼ 93%. It is also shown that the normalized length of the ground heat exchanger varies linearly up to an effect coverage of 60% where it is equal to 75% of the value encountered for an effect coverage of 100%